1. Field of the Invention
The present invention relates to a lens unit, a lens barrel for use in the lens unit, and an optical device, a scanner, and an image forming apparatus using the lens unit.
2. Description of the Related Art
Lens units having a plurality of lenses in a lens barrel have been used in various optical devices. In such a lens unit, a space between, the lenses is hermetically-sealed by the lens barrel. If this space is hermetically-sealed, when the temperature or humidity of the use environment of this lens unit is changed, air in this space expands or contracts to exert an external pressure on the lenses, resulting in, for example, deterioration of surface accuracy of an effective optical range. Moreover, when the temperature of the use environment is decreased, moisture in this space is saturated, thereby causing condensation on the surface of the lens on the space side, which is likely to influence optical performance.
In recent years, demand for high-accuracy and inexpensive lenses for scanners has been increasing. For example, when objective lenses for a scanner have a space interposed therebetween, problems such as deterioration in surface accuracy of the lenses and condensation as explained above are not negligible.
To overcome the problems, Japanese Patent Application Laid-Open No. 2002-221649 discloses a lens unit in which a communicating passage is provided between a space between a plurality of lenses and outside air.
However, in the conventional lens unit, although the space between the lenses is successfully released from a hermetically-sealed state, light from outside the lens barrel enters the effective optical range of the lenses through a communicating passage. When external light enters the effective optical range, the external light is diffused on the lens surface, thereby causing a phenomenon, such as flare, on an image formation plane. This causes deterioration of optical characteristics. Moreover, to prevent entrance of the external light, the sire of the lens barrel is necessitated to increase, resulting in an increase in the sue of the optical devices.